There has been known a control device for a continuously variable transmission having a high limiter control function that inhibits shift of a transmission ratio to an upshift side from a certain transmission ratio with the aim of performing speed-change along driver's intention without a complicated shifting operation (for example, Patent Document 1).
In a case of the related art device, however, when issuing a command of a downshift toward a Low-transmission ratio by a high limiter by an operation of a high limit control, although a primary pressure is in a pressure-release direction, a low primary pressure region existing around a primary minimum pressure, where the primary pressure passes through due to the release of the primary pressure, overlaps with a dead region (or a no-reaction region) for a primary pressure solenoid. Further, a shift by the high limiter is a relatively gentle downshift along a shifting line.
In a normal downshift, in order to increase flexibility for execution of a shift control, the dead region of the primary pressure solenoid is used. In principle, the primary pressure is determined so as to balance with a secondary pressure of a required capacity for an input torque. As a scene in which the dead region is used in the normal shift control, it is a downshift Low command with a small input torque or a time of the shift. As this driving scene, it is a coast-down or an extremely low opening degree R/L travel (Road/Load travel) etc.
When explaining each driving scene, during the coast-down, as a belt protection control, the secondary pressure is increased with a torque from a driven side being an input in addition to an input torque from an engine. As a result, the primary pressure balancing with the secondary pressure is increased, then the primary pressure does not enter the dead region.
Regarding the extremely low opening degree R/L travel, although it uses the dead region, since a shift response is slow or dull, by decreasing an F/B gain, hunting does not occur.
Accordingly, in the normal shift control, by identifying the scene, whether the primary pressure enters the dead region or not is detected. And, even in a case where the primary pressure enters the dead region, by decreasing the F/B gain, the hunting is prevented, then the control can be executed without causing a problem.
On the other hand, even during the high limiter operation, it is conceivable that, in the same manner as the normal shift control, even in the case where the primary pressure enters the dead region, the F/B gain is decreased, then the dead region can be used. However, in this case, the following difference arises.
Depending on an operation condition of the high limiter, there is no other choice except to decrease a primary pressure command in a region that is different from an originally intended region, and the dead region is used in an indefinite region depending on the high limiter. That is, unlike the normal shift control, since the scene cannot be identified, even in the case where the primary pressure enters the dead region, the F/B gain cannot be decreased.
Because of this, in a scene in which the primary pressure is further decreased from a vicinity of the solenoid dead region, the primary pressure becomes unstable due to a hydraulic pressure control in the solenoid dead region. Then, in a feedback control of the transmission ratio, due to the fact that the unstable primary pressure is recognized as disturbance, the primary pressure varies up and down and the hunting occurs. As a consequence, as a problem, hunting of the transmission ratio occurs by and according to the hunting of the primary pressure.